Magnetic materials generally fall into two classes, magnetically hard substances which may be permanently magnetized, and soft magnetic materials whose magnetization may be reversed at relatively low applied fields. Permeability and coercive filed values are a measurements of the ease with which a magnetic substance can be magnetized or carry a magnetic flux. Permeability is indicated by the ratio of B/H. The coercive force, H.sub.c, is the magnetic force or field intensity necessary to change magnetic induction B from - to +. It is important in soft magnetic materials that energy loss, normally "core loss" is kept to a minimum whereas in hard magnetic materials it is preferred to resist changes in magnetization. High core losses are therefore characteristic of permanent magnetic materials and are undesirable in soft magnetic materials.
Soft magnetic core components are frequently used in electrical/magnetic conversion devices such as motors, generators and transformers and alternators, particularly those found in automobile engines. The most important characteristics of ferromagnetic soft magnetic core components are their maximum induction, magnetic permeability, and core loss characteristics. When a magnetic material is exposed to a rapidly varying magnetic field, a resultant energy loss in the core material occurs. These core losses are commonly divided into two principle contributing phenomena: hysteresis and eddy current losses. Hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces within the iron core component. Eddy current losses are brought about by the production of induced currents in the iron core component due to the changing flux caused by alternating current (AC) conditions.
Conventional practice has been to fabricate soft magnetic materials and parts by forming laminated structures of thin die stamped ferrous sheets, typically a silicon-iron alloy. The sheets are oriented parallel to the magnetic field to assure low reluctance. The laminations must be stacked in correct alignment and the stack of laminations must then be secured together, for example, by welding, riveting, gluing, etc. The sheets may be varnished phosphated or otherwise coated to provide for some insulation between them. This insulation is intended to prevent current from circulating between sheets and therefore to keep eddy current losses low. In die stamping, there is however, a certain amount of scrap loss and hence unnecessary expense. In addition, the stamping process sometimes results in burrs requiring a subsequent deburring step and a thick bed coating to keep the sharp edges from cutting the insulation on the electrical conductors. Moreover, the stacked cores are known to suffer from large core losses at higher frequencies and are acoustically noisy (hysteresis) since the laminations tend to vibrate. This vibration also contributes to energy loss. U.S. Pat. No. 3,670,407 to Mewhinney et al. describes a stator made by such a stacked lamination and an attempt to reduce the eddy currents therein.
Another significant drawback to making soft magnetic parts from steel laminate structures is that it is difficult and time consuming to make parts having a three-dimensional configuration for moving flux out of the plane of the lamination. Certain three-dimensional configurations are very difficult and expensive to achieve with steel laminate structures.
The use of powdered metals avoids the manufacturing burden inherent in laminated structures and provides for a wider variation in the shape of the part. These materials made from consolidated powdered metals have however generally been limited to being used in applications involving direct currents. Direct current applications, unlike alternating current applications, do not require that the iron particles be insulated from one another in order to reduce eddy currents. Hence, various attempts have been made in the past to form magnetic materials from powders having the desired characteristics necessary for expanded applications including alternating current. For example, U.S. Pat. No. 3,245,841 to Clarke et al. describes a process for producing steel powder by treating the powder with phosphoric acid and chromic acid to provide a surface coating on the steel particles of iron phosphate and chromium compounds. This process however results in poorly bonded material with relatively poor insulating properties. The use of hexavalent chromium in these processes posses a significant health risk since it is carcinogenic. Hence, expensive waste treatment systems must also be employed.
In U.S. Pat. No. 4,602,957 to Pollock, et al., iron powders are treated with oxidizing agents such as potassium or sodium dichromate prior to compaction. The compact is then partially sintered at 600.degree. C. These partially sintered compacts are reported to have increased resistivity and decreased hysteresis losses when compared to bulk iron compacts. The step of sintering the part following compaction, is however, necessary to achieve satisfactory mechanical properties in the part by providing particle to particle bonding and hence strength. However, sintering increases manufacturing complexity and adds to the cost of the finished powder metallurgy part. In addition, sintering causes volume changes and results in a manufacturing process with poor dimensional control.
In other known processes to minimize eddy current losses in ferrous parts made by powder metallurgy, soft iron particles are coated with thermoplastic materials before pressing. U.S. Pat. Nos. 4,947,065 to Ward et al. and 5,198,137 to Rutz et al. teach such methods whereby iron powders are coated with a thermoplastic material. This plastic, in principle, is intended to act as a barrier between particles to reduce induced eddy current losses. However, in addition to the relatively high cost of these thermoplastic coatings, there is a considerable further disadvantage to coating the iron powders with plastic. Specifically, plastic has poor mechanical strength compared to the bulk alloy especially at high temperatures and has a tendency to creep. Thus, as a result parts made using plastic-coated iron typically have relatively low mechanical strength. Additionally, many of these plastic-coated powders require a high level of binder when pressed. This results in decreased density of the pressed core part and, consequently, a decrease in magnetic permeability and lower induction (B). Further, this material is normally pressed in a Hot Die resulting in a costly and complex manufacturing process.
Another major drawback exists with these thermoplastic-coated powders. The plastic coatings begin to degrade in the 150-200.degree. C. range, and typically melt or soften at temperatures in the 200-250.degree. C. range. Thus, the applications in which parts made from iron particles coated with thermoplastics can be used are limited to near ambient temperature, low stress applications for which dimensional control is not critical. Furthermore, it is generally not possible to achieve the stress/strain relief benefits of high-temperature annealing i.e., annealing at temperatures in excess of about 150-300.degree. C., with parts made using thermoplastic-coated iron particles. These limitations and disadvantages are also generally true for other known (typically polymeric) coatings for ferrous powders such as, for example, epoxies, phenolics, etc.
Hence, there is an important need in the industry for ferromagnetic powders to produce magnetic parts, particularly soft magnetic parts, that are well bonded (increased green strength, are tolerant of higher temperatures, have good mechanical properties, are configured in relatively complex three-dimensional shapes and have low core loss. There is a particular need for such insulated powders, and parts made therefrom, fabricated by a cost-effective method that provides precise dimensional control. Moreover, there is a need for such powders and parts made therefrom wherein the insulating properties of the coatings do not substantially degrade at relatively high temperatures. There is an additional need for processes for making these powders and the parts made therefrom that results in a highly precise net shaped part.